I. Field of the Invention
The invention relates to a novel process for the continuous electrochemical desalination of aqueous salt-containing solutions and to a device in the form of a spiral wound module for carrying out this process.
II. Description of the Related Art
It has been known for some time that aqueous solutions can be desalinated by means of electrodialysis. In this process, ion-selective membranes are introduced between two electrodes, two membranes each, a cation-selective and an anion-selective membrane, and spacers in each case defining a dilution chamber or a concentrate chamber. An ion exchange material is often additionally introduced into these chambers. If water flows through the individual chambers, anions and cations migrate according to their charge, owing to an external electrical potential which is applied to the electrodes. Owing to the ion migration, a volume stream depleted in ions (diluate) is obtained in the dilution chamber and a volume stream enriched in ions (concentrate) is obtained in the concentrate chamber. As a rule, water-impermeable anion and cation exchange membranes are arranged alternately between the electrodes connected to a direct current source. In a particular embodiment, a dilution chamber and a concentrate chamber are spiral-wound around one of the two electrodes, the second electrode surrounding the whole wound apparatus. As a result of the wound arrangement, a dilution chamber and a concentrate chamber having an approximately spiral cross section are defined. Such wound apparatuses are referred to in the technical language as “spiral wound modules”. Spiral wound modules are described, for example, in U.S. Pat. No. 4,225,413 and EP-A-O 570 341.
A disadvantage of the previously known desalination processes is that the flow inside the wound dilution chamber is tangential, i.e. along the spiral shape formed by the dilution chamber, from the outer end of the dilution chamber to its inner end (or vice versa), because, in the prior art, the feed of solution to be desalinated and the removal of the desalinated water are carried out at the inner and outer ends of the dilution chamber of the spiral wound module (or vice versa). In the dilution chamber, this results in relatively long flow distances with associated high resistances to flow.